For implementation of high-capacity optical transmission, only increasing the number of multiplexed wavelengths and constructing new optical fiber networks do not sufficiently meet demand. Thus, in recent years, rapid development has proceeded in transmission rate per transponder from conventional 10 Gbps to 40 Gbps, 100 Gbps, and 400 Gbps. For a method of increasing the channel capacity of a submarine cable, an upgrade scheme is common, in which a conventional optical fiber is used and a part of a 10 Gbps band in use is freed up by being taken up by new 40 Gbps and 100 Gbps by migration and is effectively used, or a band that is available in the first place is effectively used.
Due to such technical trends, a wavelength multiplexing optical transmission system including transponders having different transmission rates has been increasing in recent years.
Meanwhile, the wavelength multiplexing optical transmission system has a system configuration in which when trouble has occurred in a certain single wavelength, in order to protect client signals which are transmitted carried on the wavelength, a redundancy system is provided so that switchover between the redundancy system and a working system can be performed.
For example, Patent reference 1 describes an optical redundant switching device (wavelength redundancy device) including optical couplers 11-1 to 11-n that branch signals of n channels received from external devices into two directions for working systems and for a protection system for each channel; transponders 12-1 to 12-n that convert the signals for the working systems which are branched by the optical couplers 11-1 to 11-n into optical signals with λ1 to λn, and send out the optical signals; an optical switch 13 that selects a protection system signal from the signals for the protection system which are branched by the optical couplers 11-1 to 11-n, and sends out the selected protection system signal; and a transponder 12-(n+1) that converts the protection system signal sent out from the optical switch 13 into an optical signal with λn+1 and sends output the optical signal (see FIG. 1 of Patent reference 1).
When a failure is detected in one wavelength among the optical signals with λ1 to λn sent out from the transponders 12-1 to 12-n, the optical switch 13 selects a signal for the protection system corresponding to a signal carried on the wavelength where the failure is detected, and outputs the signal for the protection system to the transponder 12-(n+1). The transponder 12-(n+1) transmits the outputted signal using λn+1 (see FIG. 4 of Patent reference 1).